This invention relates generally to the filtering of multilevel data and more specifically to the utilization of a finite impulse response (FIR) binary filter in combination with an encoder for encoding non-return to zero (NRZ) into multilevel data. The present invention is particularly, but not exclusively, suited for use as a splatter filter which limits the frequency spectrum of the data before same is applied to modulate a signal in a communications system.
It is known to use a conventional analog filter as a splatter filter to reduce the transmitted spectrum of data. However, an analog filter causes intersymbol interference due to group delay variations, i.e., the non-linear phase response of the filter. This causes energy from one bit to be smeared to the next bit thereby reducing the ability to distinguish between bits. Although Bessel type filters provide a linear phase response, they provide insufficient high frequency attenuation.
A digital filter having a linear phase response such as an FIR filter can be utilized to overcome the intersymbol interference associated with analog filters. A conventional FIR filter comprises a tapped delay line in which each tapped output is weighted prior to being summed together. This method is easily applied to binary NRZ data by using a shift register made of flip-flops for the delay line. However, such an FIR filter cannot be directly used to filter multilevel data since the flip-flops only have a binary output. To directly filter multilevel data having N levels, a multilevel delay line having N states would be required. The complexity associated with such a multilevel delay line makes this approach undesirable.